Source URL does not include language prefixes

Problem/Motivation

When viewing the printable version of a language-prefixed page, the "Source URL" provided to the template does not include the language prefix.

Steps to reproduce

Proposed resolution

The Source URL variable is being constructed manually by concatenating the base URL and the source path alias. Instead, we should use the internals of Drupal's Url object, which is language and alias aware.

Remaining tasks

User interface changes

API changes

Data model changes